An increasing number of recent signboard ads feature logos and patterns of vivid colors and sophisticated designs, as well as photographic images of products and human faces. In addition, a considerable number of signboards are now large in size, so that they can give strong impact on the viewers.
Traditionally, signboard ads are produced by cutting out colored sheets in the shapes of letters and attaching them to generate logos, or by using various types of printing presses to create photographic images, in general. However, these production methods present problems in that they require a lot of time and cumbersome steps, as well as large-scale equipment such as printing presses.
Accordingly, attempts are made to utilize the inkjet method, which allows designs created on a personal computer to be printed directly on a base material, to make it easier to produce signboards featuring vivid images.
One feature of the inkjet method is that it accommodates a wide range of materials to be used as base materials for printing, thus providing a convenient way to print on sheets made of both hard and soft materials such as paper, polymers and metals. Particularly when printed signboard ads are installed outdoors and thus have a range of performance requirements including light weight, excellent strength and durability, resistance to rain, and affordable cost, the inkjet method presents a great advantage in that it makes it easy to print on polymer sheets that have these properties.
In addition, recent years have seen the emergence of super-wide-format inkjet printers having a printing width of 2,000 mm or even greater, and these inkjet printers permit production of large-size printed matters with a single swoop, instead of printing smaller segments and attaching them together as before, and this is one of the reasons that are making signboard production much easier.
In general, tarpaulins are commonly used as polymer sheets for signboard ads. It should be noted that a tarpaulin is a composite sheet produced by laminating the top and bottom of a polyester or polyamide core material with a polyvinyl chloride polymer, ethylene-vinyl acetate copolymer, or other vinyl polymer.
Among the inkjet ink compositions used for printing on these composite sheets are non-aqueous inkjet ink compositions based on organic solvents (or more recently, environmentally-friendly organic solvents). Non-aqueous inkjet ink compositions require use of materials having good wettability, drying property, fixing property, etc., with respect to polyvinyl chloride polymers, ethylene-vinyl acetate copolymers, and other vinyl polymers that are used as the surface materials for composite sheets.
Known methods to meet this requirement include using alkylene glycol monoether monoester and cyclic ester as an organic solvent (refer to Patent Literature 1), using a vinyl polymer as a binder resin, and an organic solvent that contains a specified quantity of environmentally-friendly polyalkylene glycol dialkyl ether (refer to Patent Literature 2), and blending specified quantities of diethylene glycol ethyl methyl ether and propylene carbonate as an organic solvent (refer to Patent Literature 3).
In recent years, however, the demand for higher printing speeds has given rise to the problems of insufficient filling of solid areas (hereinafter referred to as “solid fill”), poor discharge stability and mottling, as well as the problem of solidification of ink in the waste tube, when conventional non-aqueous inkjet ink compositions (especially non-aqueous inkjet ink compositions using environmentally-friendly organic solvents) are printed.
In addition, prior art is associated with maintenance problems, because when the ink remains in the printer for an extended period of time, the ink deposits on the maintenance members of the printer become dry and break away, to accumulate in, and consequently clog, the waste tube, etc.